Apple is more worried about Facebook as a competitive threat than any other tech company, according to a source who works with Apple.

Our source was surprised since it would make more sense for Apple to worry about other companies. Google has Android. Microsoft dominates the PC business. Amazon is threatening to enter the tablet space.

But Facebook is coming on like a freight train in a space Apple doesn't really understand and it's not willing to play nicely with Apple.

Another source who works with Facebook backed this up earlier this year, telling us that Apple hates Facebook because it worries Facebook could co-opt the iOS platform thanks to its strong user base.

A few other reasons we think Apple would fear Facebook:

It was burnt once by Google, so it learned its lesson: When Apple negotiated with Google to get Maps and search on the iPhone, it thought it was starting a great partnership. That didn't last, as Google built Android, which is now beating the iPhone in market share. Facebook is equally ambitious, and there's reason to believe it's going to build its own mobile OS.

Apple doesn't do social (or the web) particularly well: Apple understands the PC business, and therefore Microsoft. It gets the mobile business and Google. It gets Amazon's entry into tablets and its digital media business. Social? Apple doesn't get that at all. It doesn't really do social.

Facebook has 650 million users and growing. Just like Google is worried about Facebook fencing a big portion of the world's population, Apple should be worried about Facebook controlling hundreds of millions of people. It's a platform war out there, and Facebook is in a good position.

Whatever the case may be, we're guessing something happened when Apple was negotiating with Facebook over Ping that opened its eyes and now it's keeping its eyes on Mark Zuckerberg.